T Lymphocytes (T Cells): Subsets and Roles in Adaptive Immunity

T lymphocytes (T cells) are central to cell-mediated adaptive immunity. They originate in bone marrow, mature in the thymus, and circulate through blood and lymphoid organs. Each T cell expresses a unique T-cell receptor (TCR) recognizing specific peptide-MHC complexes.

Major Subsets and Functions

1. Cytotoxic T Cells (CD8+)

Recognize antigen presented on MHC class I (on all nucleated cells). Kill infected or malignant cells via perforin/granzyme-induced apoptosis and Fas-FasL pathway. Critical for viral clearance and tumor surveillance.

2. Helper T Cells (CD4+)

Recognize antigen on MHC class II (on professional APCs: dendritic cells, macrophages, B cells). Secrete cytokines to orchestrate immune responses:

• Th1: Activates macrophages, promotes cell-mediated immunity (IFN-γ).
• Th2: Helps B cells produce antibodies (IL-4, IL-5, IL-13).
• Th17: Defends extracellular bacteria/fungi at mucosal barriers (IL-17).
• Tfh: Essential for germinal center reactions and high-affinity antibodies.

3. Regulatory T Cells (Tregs, CD4+CD25+FoxP3+)

Suppress excessive immune responses, maintain self-tolerance, prevent autoimmunity. Deficiency causes IPEX syndrome.

4. Memory T Cells

Long-lived (central memory T_CM, effector memory T_EM, tissue-resident T_RM). Provide rapid, robust secondary responses upon re-exposure. Basis of vaccine-induced immunity.

Clinical Relevance

HIV targets CD4+ T cells. Checkpoint inhibitors (anti-PD-1/CTLA-4) reinvigorate exhausted T cells in cancer. CAR-T therapy engineers T cells to target tumors.